With known methods of fluidized combustion, properly prepared fuel (coal) is led onto the fluidizing bed on which in fluidized state it will be either combusted or it becames a gaseous media. The temperature of the fluidizing bed and thus the heat quantity to be released in the fluid bed will be determined partly by the melting properties of the coal-ash, partly by ecological requirements (removal of sulphur, arising from nitrogenoxide etc.) Released heat will be utilized mostly for evaporating energetic working media, heating feed-water or air, and preparing hot water.
With fluidized combustion, in order to increase yield of heat release, instead of the so-called buffled fluidizing bed (material of bed and most part of the coal remain on the fluidizing bed), the so-called circulated fluidization used to be applied, with which prior to final combustion, fuel (coal) is subjected to repeated separation and process of leading back onto the fluidizing bed. A drawback of the aforementioned circulated combustion is that complicated apparatuses are required, and the utilization of heat transferring surfaces relatively low.
With the known solution of fluidized combustion fuel (coal) used to be fed above the distributor assuring equal air distribution. In dependence of the speed of fluidization (1-3 m/s) the burble-fuel forms the mixture of the bed-forming material, fuel, air, combustion product, the said mixture to be found above the fluidizing bed, while in the case of a quick fluidization (2-14 m/s) fluidized combustion is taking place including material flow leaving the fluidizing bed. Quick fluidized combustion used to be realized by establishing circulation-firing, as full combustion can be obtained only by repeated circulation. With this solution usually immediately after the direct combustion chamber, a separately arranged separating high temperature fan is inserted for recycling the not fully combusted coal into the combustion chamber. Full combustion of fuel (coal) becomes possible--in dependence of quality--after having performed circulation of about 5-20 times.
Realization of fluidized combustion using conventional methods has several disadventageous features.
In the course of realizing circulated fluidization combustion, a separately arranged precipitator must be operated at high temperature (800.degree.-900.degree. C.). Such a precipitator is a most complicated prece of equipment and use of special materials will be required.
As for utilization of heat, the fan is less useul, as either it is without cooling or heat transfer is low. From the point of firing, circulated fluidization combustions have several disadvantages, such as an air requirement needed for combustion of fuel (coal) could be met only with difficulty.